1. Technical Field
The disclosure relates to a heat release rate waveform calculation apparatus and heat release rate waveform calculation method for calculating a heat release rate waveform in a spark-ignition internal combustion engine and, more particularly, to a technique for obtaining a heat release rate waveform by focusing on a period from ignition of air-fuel mixture to timing at which a heat release rate is maximum (in this specification, this period is referred to as first combustion period).
2. Description of Related Art
Generally, in order to express a combustion state of an internal combustion engine, a heat release rate in a cylinder is approximated by the use of a Wiebe function. The Wiebe function is able to suitably express a heat release rate waveform by specifying a plurality of parameters, and is utilized to estimate a heat release rate, a mass fraction burned, or the like, in combustion of an internal combustion engine.
For example, in a method of determining the parameters of the Wiebe function, described in Japanese Patent Application Publication No. 2007-177654 (JP 2007-177654 A), a shape parameter m of the Wiebe function is identified by the use of a predetermined expression on the basis of a fraction burned at a crank angle at which a heat release rate is maximum. The other parameters, that is, k, a/θpm+1 and θb, are also respectively identified by the use of predetermined expressions, and then the Wiebe function is determined so as to be adapted to an actual heat release pattern with high accuracy.
JP 2007-177654 A describes as follows. The plurality of parameters m, k, a/θpm+1, θb are identified in that way, and then work for determining the Wiebe function is performed for each of various operating conditions. Thus, the relationship between these parameters and operating parameters (load factor, rotation speed, air-fuel ratio, spark timing, and the like) of an internal combustion engine is acquired. JP 2007-177654 A further describes that, when the thus acquired relationship is utilized, it is possible to determine the Wiebe function for any operating condition of the internal combustion engine and express the combustion state of the internal combustion engine with high accuracy.
However, JP 2007-177654 A does not specifically describe a method of determining the relationship between the parameters m, k, a/θpm+1, θb of the Wiebe function and the operating parameters of the internal combustion engine. Therefore, actually, the parameters m, k, a/θpm+1, θb need to be identified for each of almost all the operating conditions, and then the Wiebe function needs to be determined for each of the operating conditions. That is, there is room for further reducing man-hours for generating heat release rate waveforms and reducing cost in the existing method.
With the above-described method, the entire heat release rate waveform is not expressed until parameters m, k, a/θpm+1, θb are identified and then the Wiebe function is determined, and, after that, the combustion state is allowed to be evaluated on the basis of the expressed entire heat release rate waveform. Thus, it is not possible to estimate and evaluate only, for example, a first combustion period (a period from ignition of air-fuel mixture to timing at which the heat release rate is maximum) with a simple method without expressing the entire heat release rate waveform.